Energy

Understanding Energy

Energy can be stored in different ways: kinetic (movement), gravitational potential (height), elastic potential (stretching), chemical (bonds), thermal (heat), nuclear, magnetic and electrostatic. Energy is transferred between stores by mechanical work, electrical work, heating or radiation. The principle of conservation of energy states that energy cannot be created or destroyed — only transferred or dissipated to less useful stores (usually thermal, via friction or air resistance). Efficiency measures how much input energy ends up in the useful output store rather than being wasted. Power is the rate of energy transfer.

Key formulas

• KE = ½ × m × v²
• GPE = m × g × h
• EE = ½ × k × e²
• E = m × c × ΔT
• Efficiency = useful / total
• P = E / t

Worked example

A 60 kg sprinter accelerates from rest to 8 m/s in 4 seconds. Calculate her kinetic energy at top speed and the average power developed.

Step 1: KE = ½ × m × v² = ½ × 60 × 8² = 30 × 64 = 1920 J.

Step 2: P = E / t = 1920 / 4 = 480 W.

Answer: 1920 J of kinetic energy, average power 480 W.

Common mistakes to avoid

• Forgetting to SQUARE the velocity in KE = ½mv² (a common 2-mark loss).

• Using g = 10 in some questions and 9.8 in others — AQA accept either but be consistent within a question.

• Confusing power (W) with energy (J).

• Using extension in cm or weight in g — always convert to SI units (m, kg).

Exam tips

• Always write out the formula first, then substitute. This earns a method mark even if arithmetic fails.

• Watch for unit conversions in mass (g → kg) and extension (cm → m).

• When asked about wasted energy, name the store: usually 'thermal energy in the surroundings'.

• Required practical: specific heat capacity of a metal block — know the method.